How are cells of the nervous system determined?

What connects the spinal cord and brain?

Upper (pyramidal) motoneurons.

Where are spinal chord signals transmitted to?

Lower motoneurons which connect to muscles.

How are signals/nerve impulses transmitted?

Action potential

Main structural divisions of the nervous system

central nervous system and peripheral nervous system

central nervous system

Includes brain and spinal cord. Acts as the main control and processing centre

peripheral nervous system

Consists of the networks of nerves branching throughout the body, carrying signals to and from the CNS.

What do the PNS and CNS together allow?

The body to sense, react, and coordinate complex functions

Four main lobes of the brain

Frontal lobe, parietal lobe, temporal lobe, occipital lobe

Frontal lobe

Involved in decision making, movement, planning, and personality

Parietal lobe

Processes touch, spatial awareness, and sensory information

Temporal lobe

Important for hearing, memory, and language

Occiptal lobe

Responsible for visual processing

Soma

Cell body of the neuron. Contains the nucleus and is responsible for maintaining the cell's functions.

Dendrites

Extend from the soma. Branching structures that receive signals from other neurons.

Axon

Long projection that carries electrical impulses away from the cell body and towards other cells.

Axon terminals

At the end of the axon. Form connections with other neurons or target tissues and release chemical messengers to communicate across synapses.

Types of neuron

~50

What does a neurons specialised shape determine?

How they receive and transmit impulses.

How does mammalian development start

When a sperm fertilises an egg, forming a zygote that undergoes rapid, successive cell divisions known as cleavage. These divisions form a solid ball of cells called the morula, which then develops into a fluid filled blastocyst consisting of an inner cell mass that will form the embryo and an outer layer of cells that will contribute to supportive tissues.

Where does the nervous system arise from

The neural ectoderm which later folds and specialises to form the brain, spinal cord, and peripheral nerves.

Neurulation

Neuroectodermal tissues differentiate from ectoderm and thicken into the neural plate. The neural plate bends dorsally with the two ends joining. Those form the neural crest. The neural tube closes and disconnects the neural crest from the epidermis. THe notochord degenerates and only persists as nucleus pulposus of the invertebral discs.

Neural plate border

Separates the ectoderm from the neural plate.

Notochord

Long, rod-like midline structure that develops dorsal to the gut tube and ventral to the neural tube. Composed primarily of a glycoproteins core that is encased in a sheath of collagen fibres.

Where does the CNS develop from?

The neural tube

Where does the PNS develop from?

A population of migratory cells that emerge at the edges of the neural tube and disperse throughout the embryo.

Neural crest

Transient embryonic structure which appears between the Neural chord and the future ectoderm during development of the vertebrate embryo. It is a stem cell niche.

What happens to Neural crest cells after neurulation?

They migrate out of their niche and give rise to various cell populations

Shh full term

Sonic hedgehog protein

BMP full term

Bone morphogenetic proteins

How protein gradients pattern the dorso-ventral (DV) axis of the neural tube

During neural tube development, Shh and BMP create opposing signalling gradients that establish the DV axis. Shh forms a ventral-to-dorsal gradient that specifies ventral cell identities such as motor neurons. BMPs form a dorsal-to-ventral gradient that induces dorsal interneuron and sensory-related cell fates.

Where is sonic hedgehog protein secreted from?

The notochord, and later the floor plate

Where are BMPs released from?

The overlying ectoderm and roof plate

What does the balance between BMP and Shh do?

Allows cells to interpret their position within the neural tube and differentiate into the appropriate neuronal subtypes.

How do secreted signals establish the dorsal-ventral pattern of progenitor domains in the neural tube

By regulating the spatial expression of transcription factors

What determines the identity of neurons that arise from the progenitor domain?

Unique combinations of transcription factors

Brain development from the neural tube

The neural tube differentiates into distinct swellings which give rise to the brain. First three primary brain vesicles appear. As development proceeds, two of these expand and subdivide to create the 5-vesicle stage. The prosencephalon becomes the telencephalon and diencephalon, the mesencephalon remains undivided, and the rhombencephalon forms the metencephalon and myelencephalon. These vesicles ultimately mature into the major structures of the adult brain.

Three primary vesicles that appear in brain development from the neural tube

the prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain)

Neuroepithelial cells

The earliest neural cells of the developing nervous system, lining the neural tube and undergoing interkinetic nuclear migration

Interkinetic nuclear migration

Neuroepithelial cells' nuclei move between the apical and basal surfaces in synchrony with the cell cycle.

What do neuroepithelial cells do during early development?

Primarily undergo symmetric proliferative divisions, producing two identical daughter neuroepithelial stem cells. This expansion increases the stem-cell pool, providing the large number of progenitors needed for later stages of neurogenesis, when divisions become asymmetric and generate differentiated neurons and glia.

Transition to asymmetric divisions in neurogenesis

Neuroepithelial stem cells shift from early symmetric divisions to asymmetric divisions, producing one radial glial cell and one newly born neuron. The radial cell retains stem cell properties and continues to generate additional neurons and glia, while the neuron exits cell cycle and begins its differentiation and migration. This strategy allows the nervous system to produce mature neurons while preserving the progenitor pool needed for continued brain growth.

What does the transition from symmetric to asymmetric divisions in neurogenesis mark?

The beginning of active neurogenesis

Radial glial cells

Serve as key neural stem cells during cortical development, undergoing asymmetric divisions to produce a self-renewing radial glial cell and a basal (intermediate) progenitor that further amplifies neuron production.

Outer radial glial cells (oRGs)

Specialised population of radial glial cells which arise later in development in the subventricular zone (SVZ). These cells also divide asymmetrically, generating another oRG cell and a transient amplifying progenitor. This expanded lineage is crucial for producing large numbers of neurons

Where do most of the neurons that populate the superficial layers of the cerebral cortex arise from?

oRG-derived progenitors

What do radial glia give rise to through asymmetric divisions?

Astrocytes, oligodendrocyte precursor cells (OPCs). Some also contribute to the generation of ependymal cells.

astrocytes

Retain supportive and regulative roles within neural circuits. Involved in blood brain barrier and take up neurotransmitter/ K+ ions from the synaptic cleft

Oligodendrocyte precursor cells

Later mature into myelinating oligodendrocytes

Glial networks

Essential for synaptic function, metabolic support, and myelination.

Oligodendrocytes in the CNS/ Schwann cells in the PNS

ensheath axons with myelin. Oligodendrocytes myelinate multiple CNS axons, whilst schwann cells myelinate a single PNS axon segment

Microglia

Arise from cells outside the CNS. The macrophages of the CNS, become activated during infection, injury, and seizure.

Satellite cells

Surround neuronal cell bodies in peripheral ganglia, providing structural support and helping to regulate the local microenvironment. They help control nutrient exchange and neurotransmitter levels and can modulate neuronal excitability within the PNS.

Ependymal cells

Line the ventricles of the brain and the central canal of the spinal chord, forming a barrier between cerebrospinal fluid and neural tissue.